This application claims the benefit of Korean Patent Application Nos. 1999-46946, 1999-56883, and 2000-19715, filed on Oct. 27, 1999, on Dec. 11, 1999, and on Apr. 14, 2000, respectively, under 35 U.S.C. xc2xa7119, the entirety of each of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a transflective LCD device.
2. Description of Related Art
In general, liquid crystal displays are divided into transmissive LCD devices and reflective LCD devices according to whether the display uses an internal or external light source.
A typical transmissive LCD device includes a liquid crystal panel and a back light device. The liquid crystal panel includes upper and lower substrates with a liquid crystal layer interposed. The upper substrate includes a color filter, and the lower substrate includes thin film transistors (TFTs) as switching elements. An upper polarizer is arranged on the liquid crystal panel, and a lower polarizer is arranged between the liquid crystal panel and the backlight device.
The two polarizers have a transmittance of 45% and, the two substrates have a transmittance of 94%. The TFT array and the pixel electrode have a transmittance of 65%, and the color filter has a transmittance of 27%. Therefore, the typical transmissive LCD device has a transmittance of about 7.4% as seen in FIG. 1, which shows a transmittance (in brightness %) after light passes through each layer of the device. For this reason, the transmissive LCD device requires a high, initial brightness, and thus electric power consumption by the backlight device increases. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device. However, this has a problem that the battery can not be used for a lengthy period of time.
In order to overcome the problem described above, the reflective LCD has been developed. Since the reflective LCD device uses ambient light, it is light and easy to carry. Also, the reflective LCD device is superior in aperture ratio to the transmissive LCD device.
FIG. 2 shows a typical reflective LCD device in cross section. As shown in FIG. 2, the reflective LCD device includes upper and lower substrates 8 and 10 with a liquid crystal layer 12 interposed. The upper substrate 8 includes color filter layers 4a, 4b and 4c (e.g., red, green, and blue) and a common electrode 6. The lower substrate 10 to includes a switching element (not shown) and a reflective electrode 2.
Ambient light 100 passes through the upper substrate 8 and the liquid crystal layer 12 and is reflected on the reflective electrode 2. When electrical signals are applied to the reflective electrode 2 by the switching element, phase of the liquid crystal layer 12 varies. Then, reflected light is colored by the color filter layers 4a, 4b and 4c and displayed in the form of images.
However, the reflective LCD device is affected by its surroundings. For example, the brightness of ambient light in an office differs largely from that outdoors. Even in the same location, the brightness of ambient light depends on the time of day (e.g., noon or dusk).
In order to overcome the problems described above, a transflective LCD device has been developed. FIG. 3 shows a conventional transflective LCD device. As shown in FIG. 3, the conventional transflective LCD device includes upper and lower substrates 22 and 18 with a liquid crystal layer 20 interposed. The upper substrate 22 includes a color filter 104, and the lower substrate 18 includes a switching element (not shown), a pixel electrode 14 and a reflective electrode 2. The reflective electrode 2 is made of an opaque conductive material having a good reflectance and light transmitting holes xe2x80x9cAxe2x80x9d are formed therein. The transflective LCD device further includes a backlight device 16. The light transmitting holes xe2x80x9cAxe2x80x9d serve to transmit light 112 from the backlight device 16.
The transflective LCD device in FIG. 3 is operable in transmissive and reflective modes. First, in reflective mode, the incident light 110 from the upper substrate 22 is reflected on the reflective electrode 2 and directed toward the upper substrate 22. At this time, when electrical signals are applied to the reflective electrode 2 by the switching element (not shown), phase of the liquid crystal layer 20 varies and thus the reflected light is colored by the color filter 104 and displayed in the form of images.
Further, in transmissive mode, light 112 generated from the backlight device 16 passes through portions of the pixel electrode 14 corresponding to the transmitting holes xe2x80x9cAxe2x80x9d. When the electrical signals are applied to the pixel electrode 14 by the switching element (not shown), phase of the liquid crystal layer 20 varies. Thus, the light 112 passing through the liquid crystal layer 20 is colored by the color filter 104 and displayed in the form of images.
As described above, since the transflective LCD device has both transmissive and reflective modes, the transflective LCD device can be used without regard to the time of day (e.g., noon or dusk). It also has the advantage that it can be used for a long time by consuming low power. However, since the reflective electrode has the transmitting holes xe2x80x9cAxe2x80x9d, the conventional transflective LCD device has a very low light utilizing efficiency compared to either the reflective LCD device or the transmissive LCD device alone.
In the reflective mode of the transflective LCD device, incident light enters the color filter 104 and is reflected on the reflective electrode 2 and reenters the color filter 104. That is, the light passes through the color filter twice. But, in the transmissive mode, light from the backlight 16 passes through the color filter once. Thus, the color purity that users perceive varies according to the mode of the LCD device.
FIGS. 4A and 4B are graphs illustrating transmissivity with respect to the light wavelengths. The graphs are obtained by a spectrum analysis method.
As is well known, all objects have a wavelength-dependent reflectivity, and their color that an observer recognizes is determined by the wavelengths of the light reflected from or transmitted through the object. The wavelength range of visible light is about 380 nm to 780 nm. The visible light region can be broadly divided into red, green, and blue regions. The central wavelength of the red visible light region is about 660 nm, that of green is about 530 nm, and that of blue is about 470 nm.
Each pixel of the LCD device has three sub-pixels so that the reflected light is colored to red (R), green (G) and blue (B) colors and, therefore each color has a dominant wavelength band leading to a high color purity by transmitting the dominant wavelength band for a predetermined color and absorbing other wavelengths.
FIG. 4A shows the relation of transmissivity and wavelength when light passes the color filter once, that is, in the transmissive mode. The blue color filter should transmit the blue color and absorb other colors. But as shown in the graph, since transmissivity of green color is relatively high in a band around 470 nm, the green color is also transmitted through the blue color filter with the blue color.
FIG. 4B shows the relation of transmissivity and wavelength when light passes the color filter twice, that is, in the reflective mode. As known from the graph, since the lines representing each dominant wavelength are steep and distributed (spaced apart), light of the wavelengths other than the dominant wavelength band are well absorbed.
Thus, the color generated by combination of the color filters has different color purity depending on the selected mode. For example, in displaying green color, the color of the transmissive mode is lighter than that of the reflective mode.
Though the color purity varies according to the selected mode, the transflective LCD device only adopts color filters appropriate to the reflective mode, since the color purity of reflective mode is better than that of the transmissive mode. Thus, the color purity that the users feel is different according to the selected mode.
Accordingly, the present invention is directed to a transflective liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the invention is to provide a transflective LCD device that can reduce the color purity difference resulting from the color filters adapted for the device regardless of the selected mode without lowering color purity. In other words, reflected light characterized by being reflected off the reflective portions of the reflector experiences substantially the same degree of color filtering as transmitted light characterized by having passed through the transmissive portions of the reflector.
In accordance with the purpose of the invention, as embodied and broadly described, in one aspect the invention includes a transflective liquid crystal display device, including: upper and lower substrates opposing each other; liquid crystal material interposed between the upper and lower substrates; first and second electrodes, arranged in correspondence to said upper and lower electrodes, respectively, to apply an electric field to the liquid crystal material; a reflector having transmissive portions and reflective portions, the reflector being positioned between the liquid crystal material and the lower substrate; a color filter layer positioned between the upper substrate and the liquid crystal material, the color filter layer having first portions aligned with the reflective portions of the reflector and second portions aligned with the transmissive portions of the reflector, each of the second portions having a higher resin density than the first portions; and a backlight device under the lower substrate.
In another aspect, the invention provides a transflective liquid crystal display device including: upper and lower substrates opposing each other; liquid crystal material interposed between the upper and lower substrates; first and second electrodes, arranged in correspondence to said upper and lower electrodes, respectively, to apply an electric field to the liquid crystal material; a reflector having transmissive portions and reflective portions, the reflector being positioned between the liquid crystal material and the lower substrate; a first color filter layer positioned between the upper substrate and the liquid crystal material; a second color filter layer positioned between the lower substrate and the reflector; and a backlight device under the lower substrate.
In another aspect, the invention includes a transflective liquid crystal display device, including: upper and lower substrates opposing each other; liquid crystal material interposed between the upper and lower substrates; first and second electrodes, arranged between the upper and lower electrodes, respectively, to apply an electric field to the liquid crystal material; a reflector having transmissive portions and reflective portions, the reflector being positioned between the liquid crystal material and the lower substrate; a color filter layer positioned between the upper substrate and the liquid crystal material, the color filter layer having first portions aligned with the reflective portions of the reflector and second portions aligned with the transmissive portions of the reflector, the second portions being thicker than the first portions; and a backlight device under the lower substrate.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate one embodiment of the invention and together with the description serve to explain the principles of the invention.